The present invention relates to an apparatus and the method for separating a composite video signal into luminance and chrominance signals in a video signal processing apparatus, and particularly to an apparatus for separating luminance and chrominance signals and the method thereof in which a band pass filter is used to separate a chrominance signal from a composite video signal by varying its bandwith according to the amplitude of a chrominance signal and a degree of color change, thereby reducing deterioration of the picture quality.
Generally, in a composite video signal used in a television, etc., the frequencies of luminance and chrominance signals are multiplexed, so that all information have been transferred while minimizing the bandwidth. Recently, according to the consumer's desire for a high quality picture and a large wide screen display, the existing problem of picture quality deterioration that was not noticeable on a small wide screen display or from a distance becomes severe. That is, in the conventional method such as a one dimensional separation of luminance and chrominance signals, the luminance and chrominance signals are not completely separated from and mixed with each other even after passing through a band pass filter, thereby causing crosstalk. Thus, deterioration of the picture quality caused by crosstalk is noticeable. Such a problem is also generated in other frequency multiplex television systems (for instance, NTSC, PAL, and so on), but, for convenience, hereinafter the present invention is described by limiting a composite video signal in an NTSC system.
A spectrum diagram of a general composite video signal is shown in FIG. 1, and a block diagram of a conventional apparatus for separating luminance and chrominance signals is shown in FIG. 2.
A composite video signal V consists of a luminance signal Y and a chrominance signal C, and if we show an R-Y signal component of the composite video signal as "Er-y", and a B-Y signal component as "Eb-y", the composite video signal V and the chrominance signal C is represented as follows: EQU V(t)=Y(t)+C(t) EQU C(t)=Er-y.multidot.cos (2.pi.fsc.multidot.t)+Eb-y.multidot.sin (2.pi.fsc.multidot.t)
where, fsc is a carrier frequency of the chrominance signal C, and is about 3.58 MHz. The bandwidth of the luminance signal Y of the composite video signal is about 0 to 4.2 MHz, and the signals Er-y and Eb-y are modulated into signals I and Q, and transmitted with bandwidths of 1.5 MHz and 0.5 MHz, respectively. Detailed information in this respect is omitted, because it is referred to in references for the NTSC standard.
The luminance and chrominance signals that appear to be overlap according to the spectrum of FIG. 1 are interleaved in such a manner that a chrominance signal is inserted between luminance signals by this frequency interleaving. To separate luminance and chrominance signals from such a composite video signal V, a comb-filter 40 and a band pass filter 50 are conventionally used, and the common characteristic of such an apparatus for separating luminance and chrominance signals is in comprising the following luminance/chrominance signal separating process.
First, a composite video signal passes through a comb-filter and a band pass filter having a frequency band of 3.58.+-.0.5 MHz, thereby obtaining a separated chrominance signal.
Second, a separated luminance signal is obtained by removing the separated chrominance signal from the composite video signal.
However, in such a luminance/chrominance signal separating process, the following problem causing deterioration of the picture quality is generated.
If the transmitted composite video signal is monochrome, the chrominance signal components are concentrated at about 3.58 MHz in the spectrum, and a band pass filter having a frequency band of 3.58.+-.0.5 MHz is used to extract the chrominance signal component. However, as the actual composite video signal is not monochrome, but separated into various colors according to the contents of the picture, the chrominance signal is spread into the region of 3.58.+-.1.5 MHz in the spectrum. To separate such a chrominance signal, if a frequency bandwidth of the band pass filter is set to 3.58.+-.1.5 MHz, the frequency bandwidth of the luminance signal decreases to about 0 to 2 MHz and results in some loss of the high frequency component of the luminance signal, thereby deteriorating the resolution of the picture. Also, a cross-color phenomenon is generated by the high frequency component of the luminance signal being mixed in with the separated chrominance signal, thereby causing deterioration of the picture. In contrast, when the bandwidth of the band pass filter is fixed at 3.58.+-.0.5 MHz, the non-filtered chrominance signal component is included in the luminance signal, generating a dot crawl phenomenon, thereby causing deterioration of the picture.
As described above, in case of watching a small wide screen or watching from a distance, the cross color phenomenon or the dot crawl phenomenon that causes deterioration of the picture quality is not noticeable, but in the case of watching a large wide screen or watching at a near distance, the deterioration of he picture quality is noticeable. To reduce the deterioration of the picture quality by preventing these phenomena, it is an important factor how wide or narrow the frequency bandwidth of the band pass filter is, i.e. what is determined to be the value X in the case of the bandwidth of 3.58.+-.X MHz.